For the 56G and the forthcoming 112G PAM4 systems, it is no longer feasible to rely on SerDes alone to transmit data through channels up to 30dB in insertion loss to achieve the desired BER. FEC becomes mandatory to work jointly with the SerDes to achieve the post-FEC BER better than a designed target, e.g., 1e-15.
This paper starts with the less-adequate approaches adopted in the industry and with some standard bodies today for FEC capability assessment. First, a large number of data based on the 28G generation KR4 FEC performance analysis and the lab test is presented to show that simply specifying a pre-FEC BER is not enough for the FEC to help achieve the desired BER. Then a novel FEC post-processing method based on voltage bathtub curves is discussed. Recursive models for both PAM4 DFE burst error probability and KP4-FEC BER estimation are derived and applied to some system link simulation examples. The methodology should help the industry to gain better insight into the system BER requirement when FEC is applied.
